Higher Basidiomycetes mushrooms (HBM) represent a major and still largely untapped source of potent new pharmaceutical products. Of approximately 15,000 known species, 2,000 are safe for human consumption, and about 650 of them possess medicinal properties (Wasser et al., 2000; Hawksworth, 2001; Kirk et al., 2001; Wasser, 2002). Of about 650 mushroom species with known medicinal properties, only about 20 species are in use at the present (Chang, 2001). Most traditional knowledge about medicinal properties of HBM comes from the Far East (China, Japan, Korea, Russian Siberia). Many pharmaceutical substances with potent and unique properties have recently been extracted from mushrooms and have made their way all around the world. Unique anticancer medicines were prepared from these extracts such as polysaccharides lentinan, krestin, and schizophyllan (Mizuno, 1999).
Present studies suggest that HBM are probiotic, i.e., they help the body to strengthen itself and fight off illness by maintaining physiological homeostasis, restoring the body's balance and natural resistance to disease. The compounds they contain have been classified as Host Defense Potentiators (HDP), which can have immune system enhancement properties. That is one of the reasons why they are currently used as adjuncts to cancer treatments in many countries (Tomatis et al., 2001). In Japan, Russia, China, and the USA, several polysaccharide anticancer and immunomodulating agents have been developed from the fruiting body, mycelia, and culture medium of various medicinal mushrooms (Lentinus edodes, Ganoderma lucidum, Schizophyllum commune, Trametes versicolor, Inonotus obliquus, Hypsizygus marmoreus, and Flammulina velutipes) (Ikekawa, 2001).
Mushroom HDP include hemicellulose (AHCC), polysaccharides, polysaccharide-peptides, nucleosides, triterpenoids, complex starches, and other metabolites. It is believed that combinations of these products target the human immune system, and also aid neuron transmission, metabolism, hormonal balance, and the transport of nutrients and oxygen. Through a host-mediated (T cell) immune mechanism, they help the body regulate the development of lymphoid stem cells and other important defense responses (Mizuno, 1999).
Chronic myelogenous leukemia (CML) is a member of a group of diseases classified as myeloproliferative disorders, which account for 20% of all leukemias. CML is a clonal disorder that is usually easily recognized because the leukemia cells of more than 95% of patients suffering from CML have a distinctive cytogenetic abnormality, the Philadelphia chromosome (Ph), that results from a reciprocal translocation between the long arms of chromosomes 9 and 22. This translocation results in the transfer of the Abelson (Abl) oncogene on chromosome 9 to an area of chromosome 22 that includes the breakpoint cluster region (Bcr) gene. This results in the presentation of a leukemia-specific fusion gene (Bcr-Abl) which gives rise to an abnormal tyrosine kinase protein, p210 (Bcr-Abl), with increased activity (Clarkson et al., 1997; Cortez et al., 1997). In addition, the Ph chromosome is also found in a sizeable portion of acute lymphoblastic leukemia (ALL) patients (25-30%) and in a small number of acute myeloid leukemia (AML) cases (Drexler et al., 1999). Bcr-Abl expressing leukemic blasts are highly resistant to different classes of chemotherapeutic drugs. K562 cells, derived from patients with CML in blast crisis (Lozzio and Lozzio, 1975), which express p210 Bcr-Abl, have been shown to be highly resistant to apoptosis induced by many chemotherapeutic agents (McGahon et al., 1994). Overexpression of Bcr-Abl has been implicated in inhibiting apoptosis induced by cytokine deprivation, DNA damage, and a variety of chemotherapeutic drugs (Cortez et al., 1997). Thus, the Bcr-Abl fusion protein has been suggested to function as an antiapoptotic factor, and overexpression of the Bcr-Abl protein in K562 cells may, in part, account for the resistance of these cells to apoptosis, thereby leading to the accumulation of leukemic blasts in patients with chronic myeloid leukemia (Urbano et al., 1998).
Gleevec (imatinib mesylate, also known as STI-571), is being used as oral treatment for patients with CML. It is a protein-tyrosine kinase inhibitor that inhibits the Bcr-Abl tyrosine kinase.
Apoptosis, programmed cell death, is a genetically controlled ablation of cells during development. It is characterized by chromatin condensation, nuclear fragmentation, cell membrane blebbing, apoptotic body formation, and mitochondrial changes, including enhanced membrane permeability, fall of mitochondrial membrane potential (Δψm), and release of cytochrome c into the cytosol. Induction of apoptosis is the principal mechanism by which the majority of chemotherapeutic agents exert their function. Consequently, failure to undergo apoptosis is the likely mechanism mediating drug resistance in tumors.
Antitumor and anticancer properties have been studied in mushrooms. The three mushrooms which have the broadest range of action are Shiitake (Lentinus edodes), Reishi (Ganoderma lucidum) and Maitake (Grifola frondosa). Another popular mushroom is Coriolus versicolor, also known as Trametes versicolor. 
Among the main active substances found in medicinal mushrooms are:
(i) lentinan, a highly purified polysaccharide fraction extracted from Shiitake mushrooms, is an approved drug in Japan. It is generally administered by injection and has been used as an agent to prolong survival of patients in conventional cancer therapy, for example, in bowel cancer, liver cancer, stomach cancer, ovarian cancer and lung cancer; it also stimulates the production of T-lymphocytes and natural killer cells and can potentiate the effect of AZT in the antiviral treatment of AIDS;
(ii) a substance known as activated hexose-containing compound (AHCC) or 1,3-beta glucan is an active fraction found in shiitake mushrooms which has shown anti-cancer properties in some human, animal and lab studies in Japan;
(iii) polysaccharide-peptide or PSP, a proteoglycan from Trametes versicolor, also known as Coriolus versicolor, has been widely used in China as anticancer and immunomodulatory agent in the treatment of patients with cancer of the stomach, esophagus, lung, ovary and cervix;
(iv) the immunostimulating polysaccharide krestin, polysaccharide-K or PSK is a popular Japanese extract made from Trametes versicolor. PSK has been shown in several studies to help cancer patients undergoing chemotherapy, and significantly extended survival at five years or beyond in cancers of the breast, liver, prostate, stomach, colon-rectum, esophagus, nasopharynx, and lung (non-small cell types). PSK acts directly against tumor cells as well as indirectly in the host to boost cellular immunity by increasing white cell activity and increasing natural killer cell function. The list of cancers for which it is known to be useful in animals includes adenosarcoma, fibrosarcoma, mastocytoma, plasmacytoma, melanoma, sarcoma, carcinoma, mammary cancer, colon cancer, and lung cancer; and
(v) a dietary supplement prepared from extracts of Trametes versicolor is in use for general health purposes. Ethanol extracts (70%) of Trametes versicolor dietary supplement reduced LNCaP cell growth and down-regulated the levels of secreted prostate specific antigen (PSA), raising the possibility of chemopreventive potential for hormone-refractory prostate cancer (Hsieh and Wu, 2001).
We have not found in the literature any publication disclosing the activity of mushroom extracts on CML cells.
Although most bioactive substances isolated from mushrooms are high-molecular-weight (HMW) polysaccharides, our interest is in low-molecular-weight (LMW) compounds capable of exhibiting antitumor activity. We have thus focused on the search for novel compounds that induce apoptosis in CML cells and might be useful in the therapy of patients with CML.